"He wanted to know if there was a difference between HIV and HPV, so I was able to explain that those are rarely confused with each other," Gates tells an audience in the video.

Watch for yourself:

For the record: HPV, or human papillomavirus, is a very common sexually transmitted disease that often causes no symptoms but is linked to an increased risk of cancer in some people. HIV, the human immunodeficiency virus, is a sexually transmitted virus that depresses the immune system and can lead to the onset of acquired immune deficiency syndrome, or AIDS, a condition that caused 1 million deaths in 2016, according to the United Nations.

It's not clear where or when Gates was speaking when the video was taken, though a tweet from the MSNBC host Chris Hayes, whose program, "All In With Chris Hayes," shared the footage, gives us a clue: "Bill Gates says that in *both* meetings he had with Trump, Trump asked if HPV and HIV were the same thing," Hayes tweeted.